Tag Archives: troubleshooting

Questions on Radiated Emissions Pre-Compliance Testing

Following the recent webinar sponsored by Rohde & Schwarz, I received way too many questions to answer during the live presentation, so I’m following up gradually and posting my answers on The EMC Blog, now hosted by EDN.com.

I just posted my third article:

3. Questions on EMC pre-compliance testing for radiated emissions

I’ll be posting one more on the questions I received on general EMC topics some time in August.

In the meantime, I’ll be attending Doug Smith’s ESD/EMC three-day seminar next week and may write up a description with pictures. The week after that, I’ll be attending the annual International Symposium on EMC right here in Denver. I’ll be posting at least two articles on new products and other activities.

EMC Troubleshooting Parts Storage

For those of you who regularly troubleshoot EMC issues, wouldn’t it be convenient to have all your “fix-it” parts all in one place? That’s what I thought as I was wandering around JoAnns (a fabric and hobby store) with my wife. I spotted this “photo keeper” storage box and thought it would be perfect to store all the little parts, gasketing, filters, etc., for quickly implementing fixes during the troubleshooting process.

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I had a chance to use this kit in a real-life consulting application as I helped a client in Chicago recently. I transported the kit inside a large Pelican case and used a quantity of bubble-wrap to protect it from shifting around. It survived just fine and added a sense of professionalism in the client’s eyes.

Hobby stores sell many similar storage boxes and you may find several to your liking. Read on for more details…

 

Make Your Own EMC Troubleshooting Kit (Download)

I recently posted a new technical paper to my EMC web site: “Making Your Own EMC Troubleshooting Kit”. This downloadable pdf file includes a summary of the six-part series of articles on the Test & Measurement World web site in The EMC Blog. I describe the complete list of contents, plus some advice on selecting a low-cost spectrum analyzer.

Here’s the download…

Review: The ARRL RFI Book (3rd Edition)

In my never-ending quest to search out useful reference books on EMC, I recently ran into the 3rd edition of “The ARRL EMC Book” (ISBN 9780872590915). First published in 1999, the new 3rd edition was released in 2010. For those unfamiliar with the ARRL (Amateur Radio Relay League), this is the national organization representing amateur radio operators (“hams”) in the U.S. Most hams are members of the league, which also publishes a range of useful operating, design and general radio reference books. Because hams are allowed to operate their two-way radios at up to 1.5kW, on occasion, this may be the cause of local interference to poorly-designed or poorly-shielded consumer products. Thus, several years ago it was decided to publish a reference book on RFI (radio frequency interference). In this article, I’ll describe the most important content and why you might want to buy it. More…

Review: The Smart Tweezers Get Smarter

For years, I’ve looked for an affordable LCR meter to measure unknown components–especially surface-mount. I ran into the Smart Tweezers (Model ST3) a couple years ago and wrote up a short review. Since then, Canadian company Advance Devices, has updated the design (Model ST5). This model is very similar, but there are a number of improvements worth mentioning.

The Smart Tweezers in use. When measuring components mounted to PC boards, you need to realize the measurement includes all components connected to the two measurement nodes. In some cases, the component must be measured “out of circuit”. However, by adjusting the new source voltage control lower, it will avoid turning on most semiconductor junctions, enabling more accurate measurements “in-circuit”. (Photo courtesy Advance Devices.)

Most LCR meters make basic measurements, but with limited ranges or inadequate accuracy. The ST5 is calibrated to NIST standards and includes a Certificate of Calibration. In addition, most conventional LCR meters aren’t really optimized to measure today’s SM (surface-mount) components. It’s tough enough working with SM parts…let alone trying to identify them once a few parts on your workbench get mixed together. Read more…

Troubleshooting Radiated Emissions at your Bench – Part 1

What do you do after returning to your workbench with a product that has just failed radiated emissions? In this multi-part series, I’ll describe simple and low-cost ways I use to help my own clients solve these issues. Most of the time, it’s possible to set up a simple 1 to 3 meter “measurement range” and determine whether a potential fix is required, or not.

PHILOSOPHY

First, a little troubleshooting philosophy. In many cases, you’ll run into more than one emission source causing the same harmonic frequency. The result is that you might apply a fix and the harmonic will do three things – either get reduced, have no change or better yet…get larger! It won’t be until you apply fixes to ALL the sources that you’ll yield positive results. This is what makes chasing down emission problems such a joy(!), I mean “challenge”!

There is also the issue of “balloon effect”; that is, you’ll beat down one frequency, only to have one, or more, pop up higher! It’s like squeezing a balloon in the middle – both ends get bigger! Often times, this is the result of resonances within your cabling or on the PC board.

In this installment, I’ll describe some simple reference antennas I use (you’ll be surprised) as well as setting up an area on your workbench where you can troubleshoot and apply potential fixes and really see whether you’re making progress, or not.

ANTENNAS 

The antenna you select should ideally be somewhere near resonance for the frequencies of concern, however, it’s not really that critical for troubleshooting purposes. So long as the antenna is fixed in length and fixed in place on the bench, you’ll receive consistent results. During troubleshooting, it’s more important to know whether the fix is “better” or “worse” or “no change” and as long as the test setup doesn’t change, the results should be believable.

Low-cost EMC antennas I use for troubleshooting, based on television "rabbit ears" and a UHF folded dipole.

Now, EMC antennas are not inexpensive, as you might imagine, so for general troubleshooting, I tend to use a couple television antennas – a pair of “rabbit ears” and a UHF “bowtie” (with TV balun to match 50-ohm coax). If the workbench is wooden, I’ll extend the antenna to approximate resonance (if possible) and tape it down to the bench with duct tape. If the bench is metallic, I’ll find a non-conductive support and position it some distance away from (or above) the bench. I usually use a test distance of about a meter, but as long as you can see the product’s harmonics on a spectrum analyzer, you’ll be able to determine your progress. Sometimes I need to insert a low-noise wide-band preamp between antenna and analyzer.

Now, obviously, ambient signals from broadcast radio, television mobile phones and two-way radio services will tend to interfere with observing the product harmonics. You may need to bring the antenna closer or set up the troubleshooting measurement in a basement or building interior away from outside windows. I usually record the known harmonics of concern and try to characterize them in relation to other nearby ambients.

CURRENT PROBES

If I know that one, or more, cables are the dominant radiation source, I might use a current probe to monitor the common-mode currents flowing on the cable, rather than an antenna. This also helps reduce the ambient signals, because current probes are generally shielded against e-fields and tend to be poor antennas. I’ll attach the probe to the cable with dominant emissions, moving it back and forth along the cable to achieve a maximum, and then fix it in place.

Commercial current probes can measure rf currents flowing on I/O cables - a very typical issue for radiated emissions issues.

BENCH

Next, I’ll clear off the workbench and (assuming the product is small) find a convenient place for it where I can work on it without moving it around much. I also place reference marks on the bench with tape, so I can reposition it for repeatable measurements. At that point, I’m ready to begin the troubleshooting and fixes while watching the emission levels.

Now don’t make the mistake of assuming that a 10 dB reduction on the bench with a one-meter test distance translates to the same reduction when measured at the test facility at a ten-meter test distance! During troubleshooting, we’re likely working in the “near field” where test distance is determined by terms of 1/r squared and 1/r cubed. At ten meters, we’re likely in the “far field” (plane waves) and the distance factor is closer to 1/r. Where “r” is the test distance. You can be fairly confident, though, that a reduction on the bench will equate to some reduction at the site.

More in the next posting!